This invention relates to a method for producing a large spherical vessel and to a vessel produced according to the method. In this specification and in the claims, the term "spherical" means having the form of any portion of the surface of a sphere.
The temperature of Liquefied Natural Gas (LNG) is about -163.degree. C. This places special demands on the choice of material for a tank in which LNG is stored, on the design of the tank and on the technique used for producing the tank. Further, the tank must be self-supporting in order to minimize transfer of heat to the contents of the tank. The diameter of a typical spherical LNG-tank is 30-40 m. A tank suitable for transport and storing of LNG is usually also suitable for transport and storing of other fluids, provided that the pressure inside the tank is reasonable. Because the use of tanks for transport and storing of LNG places stricter demands, the invention is described in the following with reference to the demands placed explicitly by LNG, but this does not exclude the application of the invention for other suitable needs.
An LNG-tank is preferably made of aluminum plates, because the extremely low temperature does not negatively affect the strength of aluminum. Alternatively, also special steel alloys can be used, but this is noticeably more expensive and forming a steel plate to spherical form is more difficult than forming an aluminum plate to spherical form.
Any point on a spherical surface can arbitrarily be designated as a pole. Knowing the radius of curvature of the spherical surface, it is possible to define lines of longitude and latitude of the spherical surface relative to the pole.
Planar, rectangular plates suitable for use in construction of spherical tanks are commercially available from various sources. The largest such plate available from a particular source may conveniently be referred to as a standard plate. Such a standard plate is made by rolling as a unitary piece and is thus essentially homogeneous in composition. Even the largest commercially available standard plates suitable for construction of a spherical tank are rather small in size relative to the surface area of a large spherical tank. Accordingly at least about 100 such standard plates are needed to construct a large spherical tank.
Traditionally, a large spherical tank is assembled from commercially available standard plates by cutting each standard plate to a desired peripheral shape to form a plate blank, bending the plate blank to spherical form, and welding the spherical plate blanks together. This procedure is very demanding, because it is difficult to ensure that the bent plate blanks are indeed spherical, and deviations from the intended spherical form affect the welding procedure. Furthermore, handling procedures are noticeably more difficult when dealing with a spherical workpiece than when dealing with a plane workpiece. Most important, however, is the fact that it is difficult to weld spherical plates together and the shape and size of the plate blanks results in the length of welding joints between spherical plates being very great.
U.S. Pat. No. 3,938,363 discloses a method of forming a plate to spherical form employing a mold that comprises a lower convex die and an upper concave die. In accordance with that method, a plate of aluminum alloy is heated to a temperature of about 498.degree. C. and is placed over the lower die. The upper die is lowered onto the hot aluminum plate, and the weight of the upper die causes the plate to be formed to the desired radius.
The lower die disclosed in U.S. Pat. No. 3,938,363 is constructed of a framework of steel plates defining rectangular cells, and the cells are filled with a refractory compound. The upper surface of the refractory compound is screeded to spherical form, the upper surface of the refractory material being approximately 5 cm above the upper surface of the steel plates. The concave die is of the same general construction as the convex die and is made using the convex die as a mold.